Imaging process for forming ceramic electronic circuits

ABSTRACT

Electronic circuits are made with a ceramic body and electrical components which can be disposed in different layers. Interconnecting means can be provided which are disposed entirely within the body. The ceramic body and the electrical components are sintered to form a monolithic structure in a single firing. The electrical components are formed in layers by mixing the electrical component in a sinterable form with a radiation sensitive material which serves as a binder. After exposure to radiation in the desired pattern the layer is developed to form the desired pattern. Multiple patterns can be formed prior to firing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improved electronic circuits and the methodsof manufacture.

2. Description of the Previously Published Art

The manufacture of various types of single and multilayer ceramicelectronic packages is generally known. Several methods are embraced,for example, U.S. Pat. No. 3,189,978 teaches a method of screen printingconductive layers of desired patterns on the surfaces of green ceramicsheets which are formed separately. Desired electrical interconnectionsbetween layers of green ceramic sheets are formed by punching holes(vias) in the green ceramic sheets, and the respective holes are filledwith conductive material. The resultant sheets are stacked into alaminated body so that the conductive layers may have the desiredinterconnections. The laminated body is sintered to obtain a multilayerstructure.

Such prior art used relatively thick green ceramic sheets thusincreasing the probability of the holes for interconnections not fullyfilled with conductive material. This results in the disconnection amongconductive layers formed on the different sheets.

In U.S. Pat. No. 3,549,784 a method of screen printing insulative andconductive material alternatively on a ceramic body is described. Theproblem with this procedure is the deformation of the final ceramicstructure because of differences of the shrinkage rates among theceramic base, the ceramic and the metallic pastes during sintering. Inorder to avoid such deformation, the art proposes that on the reverse ofthe ceramic base a metallic layer is to be applied which has an area andthickness similar to the metallic layer or layers printed on the firstside of the ceramic base. These additional printing steps result in anincrease in manufacturing cost and an increase in the thickness of thefinal ceramic structure. The resolution and registration of screenprinting techniques also limit the number of layers that can be formed.

In U.S. Pat. No. 3,978,248 the ceramic paste and metal paste for screenprinting upon the green substrate are developed to exhibit the sameshrinkage upon sintering.

Other processes for forming conductive circuit patterns on or withinceramic substrates have been proposed. U.S. Pat. No. 4,540,462 teaches amethod wherein a metal sheet is bonded to one or both surfaces of aceramic sheet. A circuit pattern is formed by coating a photopolymerlayer onto the metal layer which is subsequently imaged and developedusing conventional imaging and etching techniques. This process isrestricted to a maximum of two conductive circuitry layers.

British Pat. No. 1,256,344 teaches a method of making electricalcircuits by imaging of a photosensitive composition containing a metalor a heat fusible dielectric on a substrate. The composition is firedafter each imaging process to burn off the photosensitive material andto fuse the composition to the substrate. This process has thedisadvantage of requiring multiple firings and suffers from distortiondue to differential shrinkage between the fired and unfired materialsand distortion due to repeated firing.

U.S. Pat. No. 4,336,320 teaches a method for producing cofired conductorand dielectric patterns by depositing a photoresist layer over a drieddielectric thick-film paste. The photoresist is exposed through aphotomask and the desired pattern is developed by a process whichsimultaneously etches the unfired dielectric layer. The resulting voidsin the dielectric layer are mechanically filled with a conventionalconductive paste not containing photosensitive materials. In anotherembodiment the dielectric composition contains photosensitive vehiclewhich can be imaged and developed directly. Again, the resultant voidsin the dielectric layer are mechanically filled with a conductive pastenot containing photosensitive materials. The patent is dependent upon astenciling step where the conductive material is pressed or otherwisemechanically filling voids in the dielectric with a conductive paste.Although a key feature is a reduction in the number of firings, thisprocess still requires multiple firings for fabricating multilayerstructures. Furthermore, differential sintering shrinkage between thefired and unfired materials could lead to distortions.

OBJECTS OF THE INVENTION

It is an object of this invention to provide improved electroniccircuitry with decreased dimensions.

It is a further object of this invention to provide miniaturizedmultilayer circuits having a relatively high degree of durability.

It is a further object of this invention to provide improved multilayerceramic circuits.

It is a further object of this invention to provide the capability toproduce thin layers of dielectric materials and conductive materialswhich allow the formation of other electrical components such asresistors, inductors and capacitors.

It is a further object of this invention to provide the capability tocontrol conductor and dielectric thickness to provide greaterthicknesses for improved conductivity or insulation if required.

It is a further object of this invention to produce fine features ofconductors and dielectrics which provide improved packaging density andcomponent density.

It is a further object of this invention to provide a process whichdispenses with the mechanical tooling requirements of conventionaltechnology which reduces cost and decreases set-up time.

It is a further object of this invention to provide high-resolution,repeatable registration of conductors and dielectrics which providesimproved yield, higher quality and leads to higher performance.

It is a further object of this invention to produce special featuressuch as co-axial conductors which improve reliability of operation andreduces electrical noise.

It is a further object of this invention to provide greater integrationof CAD/CAM compatability by directly increasing the automation of thecircuit construction process.

It is a further object of this invention to provide large areaproduction of circuitry outside the conventional limits of tape castingand screen printing.

It is a further object of this invention to allow the construction ofelectrical components in a monolithic structure in a single firing.

These and additional objects will become apparent as the description ofthe invention proceeds.

SUMMARY OF THE INVENTION

The present invention relates to a method of making a structure ofconductors and insulators which have a distribution of electricalelements, such as conductors, inductors, resistors and capacitorelements and which is capable of exhibiting electrical interconnections.A layer of a composition made of a sinterable material and a radiationsensitive material which serves as a binder for the sinterable materialis applied on a sinterable support or a removable support. Thesinterable material can be an insulator or a conductor. The layer isexposed to radiation in a desired pattern. The exposed composition layeris developed by removing areas not forming the desired pattern. Then thecomposite product is sintered in a single firing process to form amonolithic structure. As indicated above, the support can be removableso that it does not form part of the fired structure or the support canbe a sinterable layer which will form part of the final fired structure.

When a multipattern structure is desired the method described aboveaccording to the present invention can be used. After developing thefirst layer at least one additional pattern is formed by repeating thebasic steps of applying the layer, exposing to a pattern of radiationand developing to form a composite product. After the desired number ofpatterns are made a single, final firing is performed.

In another embodiment a multipattern structure of conductors andinsulators having an internal distribution of electrical elements suchas conductor, inductor, resistor and capacitor elements capable ofexhibiting multilayer electrical interconnections is produced by alamination technique. A series of sinterable supports are used andvarious single patterns are placed on each support and developed by theprocess of this invention. Then the various patterns on the supports arelaminated together to form a composite product with a single finalfiring after forming the laminate. Alternatively the patterns can bemade on removable supports with the supports removed prior tolamination. In this laminating technique the structure can be made tocontain vias for the electrical interconnection between adjacent supportlayers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation showing the basic components indepositing a pattern on a support.

FIG. 2 is a schematic representation showing the basic steps in buildinga multi-pattern structure on a support.

FIG. 3 is a schematic representation showing an alternative method ofbuilding a multi-pattern structure by lamination of separate supports.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed at producing either a single layer ormultilayer composite structure comprising a plurality of conductive andinsulative layers by radiation curing of compositions comprisingradiation sensitive materials and sinterable materials followed by asingle firing step to consolidate the structure into monolithic form.The sinterable materials can be either conductors or insulators.

In one embodiment a single layer structure is made. By loading ceramicand metal particulates in a radiation curable polymer, which serves as abinder for the particulates, such a composition can be formulated to beapplied to a ceramic substrate for subsequent imaging, developing andsintering. An electronic package can then be obtained characterized by arange of dielectric constants imparted to it by the ceramic particulatesand by a range of resistivities imparted to it by the metallicparticulates.

Materials suitable for the dielectric particulates include, but are byno means limited to, alumina, beryllia, silicates, titanates, otheroxides, aluminum nitride, and other nitrides and carbides. Materialssuitable for the conductive particulates include, but are not limitedto, metals such as molybdenum, silver, gold, copper, and platinum groupmetals as well as compounds such as ruthenium dioxide or tin oxide, orcarbides.

Various types of radiation curable polymers can be used. See for example"UV Curing: Science & Technology", edited by S. P. Pappas, TechnologyMarketing Corp. (1980). Many types of radiation are contemplated forimaging and scanning to produce patterns and they can include, forexample, UV, EB, IR, X-rays and lasers. Examples of ultraviolet lightcurable polymers are acrylates, methacrylates and cinnamate systems.

It is understood by those skilled in the art that the selection of theappropriate photopolymer is dependent upon the curing conditions (time,temperature, exposure to selected radiation, loading, etc), the firingconditions (rate of temperature increase, atmosphere, dwell time, peaktemperature, etc.). An example of an ultraviolet light curable polymeris ACCUTRACE 3000 made by W. R. Grace & Co. The type of polymer isdescribed in U.S. Pat. No. 4,422,914 and the entire content of thispatent is incorporated herein by reference. The polymer has a urethanestructure with terminal unsaturation and a terminal carboxylic group.The polymer can be loaded in one case with a ceramic particulate such asalumina powder to make an insulator or in another case with a metalpowder such as molybdenum to make a conductor. These compositions areformulated and applied by photo-imaging techniques to unfired ceramicsubstrates. Patterns with less than 1 mil line and spacing can begenerated with this system.

Some curable polymers may leave a carbon residue when the loadedcompositions are sintered in a reducing atmosphere to fuse thesinterable material. If a dielectric is being made, for example, thenthis conductive carbon may be objectionable. In this event the firingcan be carried out in an atmosphere conducive to the removal of carbon,e.g. an oxidizing atmosphere or a cleanly decomposing polymer system canbe utilized. If thick sections need to be cured, changes can be made inthe polymer system, the polymerization initiator system as well as inthe radiation to give selective penetration.

In another embodiment multi-pattern composite structures can also besuccessfully fabricated.

These embodiments will be described with reference to FIGS. 1, 2 and 3.

FIG. 1 is an illustration of the basic steps in fabricating a conductivepattern on a support. The support can be a removable support or asinterable support. Referring to FIG. 1A, a metal loaded photo-sensitivepolymer composition 20 is applied in the form of a coating on asubstantially continuous support 30. The composition is exposed to UVradiation through a photomask 10. After appropriate development bytechniques well known in the art, the desired pattern 21 which is anegative image of the photomask 10 is formed on the support as shown inFIG. 1B. The photo-sensitive polymer as illustrated in FIG. 1 is anegative type photopolymer. It is understood by those skilled in the artof imaging with photo-sensitive polymers that either a positive or anegative photo-material could be used and that the method of developingthe image will vary according to the photo-sensitive polymer being used.

In a preferred embodiment a metal powder such as molybdenum metal powdercan be mixed with the preferred ACCUTRACE 3000 photopolymer in apreferred weight ratio of about 72:28 under low UV lighting conditions.The blended mixture is doctor blade coated to a thickness of about 1-2mils upon an unfired ceramic substrate which was prepared according toconventional tape casting methods. See, for example, "Tape Casting ofCeramics" by R. E. Mistler, D. J. Shanefield and R. B. Runk at pages411-448 in "Ceramic Processing Before Firing", edit. by G. Y. Onoda andL. L. Hench, J. Wiley & Son (1978). The coated substrate is positioned,without contact, under a photomask. The coated substrate is illuminatedwith colluminated UV light (from a high pressure mercury arc lamp)through the photomask for about 30 seconds. The exposed coated substrateis developed in an aqueous sodium carbonate solution such as a 0.75weight percent solution. The developed pattern is subsequently rinsed,dried and post cured to enhance the integrity of the pattern. Thepatterned substrate is cofired in a reducing atmosphere at 1600° C. tosinter the metallic pattern and ceramic support.

FIG. 2 is an illustration of the basic steps in fabricating amulti-pattern structure. Referring to FIG. 2A, a metal loadedphoto-sensitive polymer composition 20 is applied in the form of acoating on a support 30. The composition is exposed to UV radiationthrough a photomask 11. After appropriate development as shown in FIG.2B, the desired pattern 22 which is a negative image of the photomask 11is formed on the support. To build a multi-patterned structure, a secondlayer of loaded photopolymer 40 which in this case can be a dielectriccomposition is applied over the previously patterned substrate and isimaged as described above utilizing photomask 12 as shown in FIG. 2C.The resulting structure 41 shown in FIG. 2D is obtained afterdevelopment of the second loaded photopolymer layer 40.

This coating, imaging and developing procedure can be repeated as shownin FIGS. 2E and 2F to create three dimensional patterned structures.Note that in FIG. 2E the layer of material designated 23 to make thenext pattern 24 does not necessarily have to be formed in a horizontalplane; the layer can be arranged to make a 3 dimensional patternstructure.

The support 30 as illustrated in FIG. 2 can be a removable or sinterablesupport.

As an example of this multi-pattern embodiment a conductor polymercomposition was made by mixing molybdenum metal powder with ACCUTRACE3000 in the weight ratio of 72:28 under low UV lighting conditions. Anadditional mixture was prepared by mixing alumina powder with ACCUTRACE3000 in the weight ratio of 73:27 under similar conditions. The blendedalumina mixture was doctor blade coated to a thickness of 3-5 mils upona removable Mylar substrate. This first layer was flooded with UV lightto polymerize the photopolymer resulting in a continuous dielectriclayer.

The metal loaded photopolymer was doctor bladed to a thickness of 1-2mils. The coated substrate was positioned, without contact, under aphotomask. The coated substrate was illuminated with colluminated UVlight (from a high pressure mercury arc lamp) through the photomask for30 seconds. The exposed coated substrate was developed in an aqueoussodium carbonate solution generating a second layer. The developedpattern was subsequently rinsed, dried and post cured to enhance theintegrity of the pattern.

A third layer was produced by doctor blading alumina loaded photopolymerto a thickness of 3-5 mils above the previously patterned layer. Asimilar imaging and developing procedure was followed generating a threedimensional structure. The dielectric pattern was chosen to contain viasto provide interconnection to the fourth layer which was generated usingthe metal photopolymer material. The multiple patterned substrate wascofired in a reducing atmosphere at 1600° C. to sinter the metallicpattern and ceramic support. The result of this single firing was amonolithic substrate demonstrating integrity of the insulative andconductive structure, and more specifically, the fine dimension andelectrical continuity of the conductive network.

This technical approach of using photo tools allows the production offiner detail than conventional screen printing and more accurateregistration of multiple layers. Conventional screen printers arelimited to a relatively small area in which accurate printing can beaccomplished. The present technology for photoimaging allows theproduction of large areas with high resolution and registration whichhas been demonstrated up to 18 by 24 inches on a production basis.

The processes produce high accuracy sequential patterns of materialswhich can be of variable conductivity or insulators with variabledielectric constants. Thus sophisticated electrical components such ascapacitors, co-axial conductors, resistors, etc. can be constructedwhich may be enclosed within the finished body.

The photoimaging process allows the production of complex structureswithout mechanical tools. By utilization of photomasks, structures suchas vias, which are vertical conductor paths, solder pads and other suchstructures can be created. This elimination of mechanical tools allowsfor the rapid changeover from one product to another and it allowsmodifications to be easily accomplished and for engineering changeorders to be rapidly instituted.

The active radiation generating the desired pattern can be fromconventional UV light sources such as high pressure mercury or xenonlamps. However, the use of scanned lasers, electron beams or X-rays forexample can also serve the same imaging purpose without the use of aphotomask. The use of photomasks or these other sources of scannedradiation allows this process to be effectively controlled by CAD/CAMsystems. This computer control allows even more effective utilization ofthe invention in creating more complex structures on an even more timelybasis. This system can be fully automated and enclosed in a compact areaproviding improved product yields.

FIG. 3 illustrates another alternative method of building amultipatterned structure. Multiple patterns are generated on separatesinterable supports. The separate supports are subsequently laminatedand cofired to form a single multipatterned structure. Referring to FIG.3A, a metal loaded photopolymer layer 20 is applied upon a sinterablelayer 30 and imaged through the photomask 14. After developing, theresultant pattern 23 is shown in 3B. This patterning can be repeated astaught in the description of FIG. 2.

Referring to FIG. 3C, a metal loaded photopolymer 20 is applied to asinterable support 31 which contains via holes for electricalinterconnection to adjacent layers. (The via holes can be formed byconventional punching or by this photopolymer process.) The loadedphotopolymer 20 is exposed through photomask 15 and developed aspreviously described to form patterned layer 24 as shown in FIG. 3D.This patterning can be repeated as taught in the description of FIG. 2.

Combinations of the patterned supports described in FIGS. 3B and 3D canthen be positioned and laminated as shown in FIG. 3E. The bottom layer32 can be one of the above layers or a differently prepared sinterablematerial. The laminated patterned supports are cofired to form amonolithic structure 33 as shown in FIG. 3F.

As an example of this embodiment, several of the patterned supportsdescribed in FIG. 1 are laminated under heat and pressure and cofired tocreate a monolithic structure.

As a further variation on this embodiment the support can be removable.Prior to the lamination step the supports can be removed and the variouslayers laminated together and then fired.

It is understood that the foregoing detailed description is given merelyby way of illustration and that many variations may be made thereinwithout departing from the spirit of this invention.

What is claimed is:
 1. A method of making a structure of conductors andinsulators having a distribution of electrical elements capable ofexhibiting electrical interconnections comprising the steps of(a)applying on a substantially continuous sinterable support a layer of acomposition comprising a sinterable material and a radiation sensitivematerial which serves as a binder for the sinterable material, saidsinterable material being an insulator or a conductor; (b) exposing thelayer to radiation in a desired pattern; (c) developing the compositionlayer by removing areas not forming the desired pattern; and (d)sintering the composite product in a single firing process whereby thesubstantially continuous support and layer shrink together to form amonolithic structure.
 2. A method according to claim 1, wherein in step(a) the substantially continuous sinterable support is provided bycoating a sinterable material and a radiation sensitive material on aremovable support and uniformly exposing the materials to radiation. 3.The method according to claim 1, wherein the sinterable material is aninsulator.
 4. The method according to claim 1, wherein the sinterablematerial is a conductor.
 5. The method according to claim 1, wherein theexposure in step (b) utilizes a photomask.
 6. The method according toclaim 1, wherein the exposure in step (b) utilizes scanned radiation. 7.A process according to claim 1, wherein a multipattern structure ofconductors and insulators having an internal distribution of electricalelements capable of exhibiting multilayer electrical interconnections isproduced, comprising(i) forming after the developing in step (c), atleast one additional pattern by applying a layer of a compositioncomprising a sinterable material and a radiation sensitive materialwhich serves as a binder for the sinterable material, said sinterablematerial being an insulator or a conductor, and repeating steps (b) and(c) to form a composite product; and (ii) conducting the single firingin the final sintering step (d) after forming the last pattern in step(i).
 8. A process according to claim 2, wherein a multipattern structureof conductors and insulators having an internal distribution ofelectrical elements capable of exhibiting multilayer electricalinterconnections is produced, comprising(i) forming after the developingin step (c), at least one additional pattern by applying a layer of acomposition comprising a sinterable material and a radiation sensitivematerial which serves as a binder for the sinterable material, saidsinterable material being an insulator or a conductor, and repeatingsteps (b) and (c) to form a composite product; and (ii) conducting thesingle firing in the final sintering step (d) after forming the lastpattern in step (i).
 9. A process according to claim 7, wherein thesinterable material is an insulator.
 10. A process according to claim 7,wherein the sinterable material is a conductor.
 11. The method accordingto claim 7, wherein the exposure in step (b) utilizes a photomask. 12.The method according to claim 7, wherein the exposure in step (b)utilizes scanned radiation.
 13. A monolithic structure made by theprocess of claim 8 with a single firing.
 14. A monolithic structure madeby the process of claim 7 with a single firing.
 15. A process accordingto claim 1, wherein a multipattern structure of conductors andinsulators having an internal distribution of electrical elements suchas conductor, inductor, resistor and capacitor elements capable ofexhibiting multilayer electrical interconnections is produced,comprising(i) after the developing of the first pattern in step (c),forming at least one additional pattern by repeating steps (a)-(c) on aseparate support; (ii) laminating together the various patterns to forma composite product; and (iii) conducting the single firing in the finalsintering step (d) after forming the laminate in step (ii).
 16. Aprocess according to claim 15, wherein vias are made through thesupport.
 17. A monolithic structure made by the process of claim 15 witha single firing.